Check out my source, which explains it better than me.
"why one form of collectivity is generally okay when the other isn't -- reference to 'white supremacy' sometimes helps. Exclusion of non-white people from all-white gatherings smacks of racism because so many examples from the past have been explicitly motivated by just that -- white supremacist desires to separate white people from the supposedly contaminating presence of racial minorities.
On the other hand, such non-white gatherings as history months, black-owned businesses, minority organizations and clubs, and even beauty contests, have often been a response to white supremacy -- a way of reasserting, and even repairing, something about a racial group that white supremacist ideology and practice has long denigrated, and damaged. Throughout the history of the U.S., and of the West more generally, the very concept of whiteness itself has been all about dominance of other people and extraction of their labor and resources. As a result, white people are still the group that's generally on top, which means that there's no corrective reason -- nor a good celebratory one -- for white people to gather together as members of their race.
And one more thing -- unlike racist all-white gatherings (such as that proposed, blockheaded basketball league that's been making the rounds recently), minority racial or ethnic gatherings are rarely exclusive -- from what I've seen, other people are often welcome. Which is not the case, from what I've also seen, when white people get together under the banner of racial whiteness."